Cancer is the second most common cause of death in the U.S., exceeded only by heart disease, and accounts for 1 of every 4 deaths. Since 1990, in the U.S. alone, nearly five million lives have been lost to some form of cancer.
For example, breast cancer affects 186,000 women annually in the U.S., and the mortality rate of this disease has remained unchanged for 50 years. Surgical resection of the disease through radical mastectomy, modified radical mastectomy, or lumpectomy remains the mainstay of treatment for this condition. Unfortunately, a high percentage of those treated with lumpectomy alone will develop a recurrence of the disease.
Lung cancer is the most common cause of cancer death in both sexes in the United States. Lung cancer can result from a primary tumor originating in the lung or a secondary tumor which has spread from another organ such as the bowel or breast. Primary lung cancer is divided into three main types; small cell lung cancer; non-small cell lung cancer; and mesothelioma. There are three types of non-small cell lung cancer: squamous cell carcinoma, adenocarcinoma, and large cell carcinoma. Mesothelioma is a rare type of cancer that affects the covering of the lung called the pleura, and is often caused by exposure to asbestos.
Ovarian cancer accounts for about 3% of all cancers among women and ranks second among gynecologic cancers, following cancer of the uterine corpus. Ovarian cancer affects over 20,000 women in the United States each year and causes some 15,000 deaths annually. If the disease is diagnosed at the localized stage, the 5-year survival rate is over 90%; however, only about 19% of all cases are detected at this stage.
The incidence of pancreatic cancer has been increasing steadily in the past twenty years in most industrialized countries, exhibiting the characteristics of a growing epidemiological problem.
Leukemia is a type of cancer that affects blood cells. Among the currently prescribed treatment regimes for leukemia are total body irradiation and chemotherapy. The two treatment regimes, however, pose a clinical dilemma: because leukemia is a cancer of the blood, all of the cells in the blood and all of the cells that arise in bone marrow must be treated in order to ensure destruction of the neoplastic cells. Destruction of all these cells leaves the patient in a severely immunodepressed state which could be as fatal as the leukemia.
Moreover, some cancer drugs are metabolized by an organism's naturally occurring enzymes such as adenosine deaminase (ADA, EC 3.5.4.4) and cytidine deaminase (CDA, also termed cytosine nucleoside deaminase, cytidine aminohydrolase, or EC 3.5.4.5). These enzymes function to deaminate natural aminopurine and aminopyrimidine nucleosides, respectively, in human and other organisms. These enzymes also convert active nucleoside-based cancer drugs into inactive metabolites. For example, the purine nucleoside drug arabinosyladenine (fludarabine, ara-A) is deaminated by ADA; the resulting compound, with the parent amino group replaced with hydroxyl, is inactive as an antitumor agent compared to the parent compound.
CDA is a component of the pyrimidine salvage pathway. It converts cytidine and deoxycytidine to uridine and deoxyuridine, respectively, by hydrolytic deamination (Arch. Biochem. Biophys. 1991, 290, 285-292; Methods Enzymol. 1978, 51, 401-407; Biochem. J. 1967, 104, 7P). It also deaminates a number of synthetic cytosine analogs which are clinically useful drugs (Cancer Chemother. Pharmacol. 1998, 42, 373-378; Cancer Res. 1989, 49, 3015-3019; Antiviral Chem. Chemother. 1990, 1, 255-262). Conversion of the cytosine compounds to the uridine derivatives usually confers loss of therapeutic activity or addition of side-effects. It has also been shown that cancers that acquire resistance to cytosine analog drugs often overexpress CDA (Leuk. Res. 1990, 14, 751-754). Leukemic cells expressing a high level of CDA can manifest resistance to cytosine antimetabolites and thereby limit the antineoplastic activity of such therapeutics (Biochem. Pharmacol. 1993, 45, 1857-1861).
Tetrahydrouridine (THU, or 1(β-D-Ribofuranosyl)-4-hydroxytetrahydropyrimidin-2(1H)-one) has been known as an inhibitor of cytidine deaminase for a number of years.
Various reports have suggested that co-administration with THU increases the efficacy and oral activity of cytidine-based drugs. See, e.g., Cancer Chemotherapy Reports 1975, 59, 459-465 and Blood 1985, 66, 527-532. However, early CDA inhibitors such as THU suffer from drawbacks that include acid instability (J. Med. Chem. 1986, 29, 2351) and poor bioavailability (J. Clin. Pharmacol. 1978, 18, 259).
5-Aza-2′-deoxycytidine (also termed decitabine; or, the active agent in the branded product Dacogen®) is an antineoplastic agent for the treatment of myelodysplastic syndrome (MDS), with potential utility for the treatment of AML and CML as well. Like the other cytidine-based drugs, its oral bioavailability and efficacy are limited by deactivation by CDA. THU has been shown to improve the potency of decitabine in a sickle cell disease model in baboons (Am. J. Hematol. 1985, 18, 283-288). In addition, another known CDA inhibitor, zebularine, has been shown to enhance the efficacy of decitabine in mice with L1210 leukemia (Anticancer Drugs 2005, 16, 301-308).
There is therefore an ongoing need for new, potent and therapeutically useful inhibitors of CDA that can be used with decitabine for treating cancer or neoplastic disease.